1. Field of the Invention
The present invention relates to an image forming apparatus having a device for detecting the concentration of a developing agent stored therein and, more particularly, to an image forming apparatus such as a laser printer or an electronic copying machine into which a developing unit can be detachably mounted.
2. Description of the Related Art
In a conventional apparatus for forming an image with a developing agent, a device for detecting the concentration of the developing agent is initialized with reference to a developing agent concentration at the time of shipping or the like. That is, at the time of shipping, the reference developing agent concentration is converted into a reference voltage signal, and this signal is permanently stored as a reference voltage value in the developing unit body. After shipping, when the actual developing agent concentration becomes lower than the reference concentration, the decrease in concentration is detected by the device for detecting the concentration of the developing agent. The device generates a detection signal having a high voltage value. When the detection signal exceeds a predetermined voltage level, a toner is replenished from a toner hopper to a stirrer. The toner is stirred by the stirrer. When the concentration of the developing agent in the stirrer reaches the reference value, replenishment of the toner is stopped.
In a conventional image forming apparatus, however, even if the developing unit at the time of shipping is replaced with a new developing unit, the detection signal level of the developing unit at the time of shipping is given as a reference, thereby controlling the developing agent concentration of the new developing unit. Since developing units are manufactured as single units, the relationships between the voltage levels of detection signals from concentration detecting devices and the developing agent densities vary depending on different developing units. When developing units having different detection characteristics are controlled on the basis of a reference value permanently stored in an image developing apparatus, the developing agent concentration varies whenever the developing unit is replaced with a new one. Therefore, the developing units cannot be optimally controlled depending on the characteristics of the individual developing units. Therefore, the reference developing agent concentration cannot always be maintained to be the reference developing agent concentration in the stirrer. As a result, the quality of the image formed by the image forming apparatus may be degraded. That is, even if the concentration of the developing agent is decreased, the voltage level of the detection signal generated by the concentration detecting device is not increased and the level of the detection signal does not reach the voltage level corresponding to replenishment. Therefore, the toner is not replenished, and the concentration of the developing agent in the developing unit is kept low. As a result, an image having a very low concentration is formed, thus degrading image quality.
When the detection signal from the concentration detecting device is kept at a level equal to or higher than the reference level, the toner is supplied to increase the developing agent concentration in the developing unit. An image having an excessively high concentration is formed, and image quality is therefore degraded.